An organic electroluminescent device has been recently attracting attention as promising to realize a display device having an extremely small thickness, a light weight, a small size, a low power consumption and the like in the future. The organic electroluminescent device is expected to be widely used in the future. In particular, the combination of the device with a low-temperature polycrystalline silicon thin film transistor realizes further reductions in thickness, weight and size. An organic electroluminescent device driven by a low-temperature polycrystalline silicon thin film transistor can be one of the ideal devices (T. Shimoda, M. Kimura, et al., Proc. Asia Display 98, 217 (1998), M. Kimura, et al., IEEE Trans. Elec. Dev., to be published).
Examples of a method of producing an organic electroluminescent element include a vacuum process and a liquid-phase process. In general, a vacuum process such as a deposition method or a sputtering method is employed for producing a low-molecular organic electroluminescent element (in the present Specification, each pixel constituting a display panel is referred to as an organic electroluminescent element).
On the other hand, letterpress printing, intaglio printing, stencil printing, or a non-plate printing method is employed for production of a high-molecular organic electroluminescent element. A liquid-phase process such as a spin coating method, a squeegee application method, an ink-jet method or a nozzle-coating method is used in the non-plate printing method. In particular, the ink-jet method where film formation and patterning for applying colors separately can be simultaneously performed is said to be advantageous.
In ink-jet method, which is a method of forming as a pattern thin films different from each other in properties on a single substrate through coating, it is necessary to provide an insulating layer as an element-separating structure between pixels, in order to prevent problems that thin film materials get mixed with each other on the substrate or that a discharged liquid material flows out into a wrong pixel adjacent to the target area. (See JP-A-2002-305077.)
The term “insulating layer” as used herein refers to a structure interposed between pixels for separating organic electroluminescent elements of respective pixels.
However, a method of producing an organic electroluminescent device including forming the insulating layer involves drawbacks as described below.
FIG. 1 shows a sectional view of a conventional organic electroluminescent device including as insulating layers a first insulating layer and a second insulating layer as described in Proc. Asia Display 98, 217 (1998), IEEE Trans. Elec. Dev., and FIGS. 2(a) to 2(d) show a method (steps) of producing the device.
The organic electroluminescent device of this conventional example includes a first insulating layer 1 and a second insulating layer 2 on an anode 3. After forming the first insulating layer 1 and the second insulating layer 2, organic electroluminescent layers 4 are formed from compounds different depending on the color which each of the layers targets by means of an ink-jet process or the like. Then, a film of cathode 5 is formed on the layers 4, whereby an organic electroluminescent device is completed.
The second insulating layer 2 is controlled to be liquid-repellent through an appropriate surface treatment, whereby in applying the organic electroluminescent layer 4 to each pixel, colors can be prevented from mixing with each other.
However, controlling the second insulating layer 2 to be liquid-repellent, which results in generation of thinner regions 6 of an organic electroluminescent layer 4 around edges of the second insulating layer 2, leads to difficulty in uniform film formation.
When the anode 3 and the cathode 5 are present in this region 6, short circuit occurs, with the result that a leakage current remarkably increases. In view of the foregoing, the first insulating layer 1 is provided so that no short circuit occurs around the edge of the second insulating layer 2.
In the process shown in FIG. 2, the anode 3 is formed of ITO, the first insulating layer 1 is formed of SiO2 through plasma enhanced chemical vapor deposition (PECVD) of ethyl silicate (tetraethoxy silane: TEOS), and the second insulating layer 2 is formed of polyimide by means of a spin coating method.
In this conventional example, the first insulating layer 1 is formed on the anode 3 formed of ITO (FIG. 2(a)). Then, patterning is performed in such a manner that openings are provided at positions on the anode 3 which are predetermined to emit light (FIG. 2(b)). Next, the second insulating layer 2 is formed by means of a liquid-phase process (FIG. 2(c)). Patterning is performed in such a manner that openings are provided at positions on the anode 3 which are predetermined to emit light (FIG. 2(d)).
As can be seen from FIG. 2, since a film having a considerably large thickness is used as the first insulating layer 1, the part of the second insulating layer 2 on the opening portion of the first insulating layer 1 is much thicker than other parts.
Therefore, in etching the second insulating layer 2, setting the etching time in accordance with a thin portion may cause an etching residue to generate at a thick portion. Reference numeral 7 in FIG. 2 (d) denotes an etching residue thus produced in an opening portion. In addition, setting an etching time in accordance with a thick portion may cause a large side-etched portion to generate at a thin portion.
In addition, since the first insulating layer 1 is formed by means of a vacuum process, the surface of the first insulating layer 1 is not flat. That is, undulations are present on the surface according to the presence or absence of the anode 3 underneath. Furthermore, nonuniformity in thickness of the second insulating layer 2 is present in correspondence with the undulations of the surface. The etching residue of the second insulating layer 2 may generate as a result of the nonuniformity in thickness. Reference numeral 8 in FIG. 2(d) denotes an etching residue thus produced on an uneven surface. When attempts are made to completely remove the etching residue 8 from the uneven surface, for example, a large side etch may generate at any other site. Thus, reduction in performance of an organic electroluminescent device which is a result of etching residue or unevenness of the surface is a big problem.
In addition, in order to improve the light-emitting property of an organic electroluminescent (EL) element, a work function or the like, in addition to the cleanliness and irregularities of the surface on which an organic electroluminescence compound is formed into a film, must be controlled to an optimum value in accordance with the organic electroluminescence compound (JP 2004-63210 A). Accordingly, procedures such as a step of liquid-washing the surface onto which the organic electroluminescence compound is to be applied or a step of removing an impurity such as an organic substance adhering to a substrate surface through an oxygen plasma treatment are required. However, it is extremely difficult to completely wash a substrate surface on which the above-described insulating layer is present or to uniformly perform a surface treatment (JP 2001-126867 A)
The reason why an organic electroluminescent device must be produced still with an insulating layer in spite of such disadvantages involved in the formation of an insulating layer is that, since a conventional organic electroluminescent device using a polymer organic electroluminescent compound basically has a laminate structure composed of an anode/a hole-injecting layer/an organic electroluminescent layer/a cathode, such an organic electroluminescent device cannot be put into practical use unless produced so as to include a hole-injecting layer interposed between an anode and an organic electroluminescent layer (JP 2000-516760 A).
Any kind of compound can be used for the hole-injecting layer as far as the compound has a function of efficiently injecting a hole from the anode to the organic electroluminescence layer. A water-soluble conductive polymer (BAYTRON (registered trademark) manufactured by Starck Vitec. Co.) has been conventionally widely used. One requirement of a compound to be used for a hole-injecting layer is that the hole-injecting layer must not be dissolved in an organic electroluminescent compound which is to be applied onto the hole-injecting layer.
Moreover, since an organic electroluminescent compound is dissolved in an organic solvent and then applied, it is preferable that a hole-injecting layer be insoluble in an organic solvent (that is, water-soluble).
In other words, in a process where an organic electroluminescent device is produced by means of an organic electroluminescent compound requiring a hole-injecting layer, it is necessary that a water-soluble hole-injecting layer be first applied onto the surface of an anode. When the surface of the anode is water-repellent, the hole-injecting layer is repelled and cannot be applied.
Accordingly, it is necessary to control apart on the surface of an electrode on which a hole-injecting layer is formed to be hydrophilic and it is also necessary to form a pattern such that a water-repellent insulating layer like a dike is formed on the part between electrodes on which part a hole-injecting layer is not to be formed. Therefore, formation of an insulating layer is conventionally indispensable.